Human influences will continue to change atmospheric composition throughout
the 21st century.

Figure 5: The global climate of the 21st century will depend on natural
changes and the response of the climate system to human activities.

Climate models project the response of many climate variables – such as
increases in global surface temperature and sea level – to various scenarios
of greenhouse gas and other human-related emissions. (a) shows the CO2
emissions of the six illustrative SRES scenarios, which are summarised in
the box on page 18, along with IS92a for comparison purposes with the SAR.
(b) shows projected CO2 concentrations. (c) shows anthropogenic
SO2 emissions. Emissions of other gases and other aerosols were
included in the model but are not shown in the figure. (d) and (e) show
the projected temperature and sea level responses, respectively. The "several
models all SRES envelope" in (d) and (e) shows the temperature and sea level
rise, respectively, for the simple model when tuned to a number of complex
models with a range of climate sensitivities. All SRES envelopes refer to
the full range of 35 SRES scenarios. The "model average all SRES envelope"
shows the average from these models for the range of scenarios. Note that
the warming and sea level rise from these emissions would continue well
beyond 2100. Also note that this range does not allow for uncertainty relating
to ice dynamical changes in the West Antarctic ice sheet, nor does it account
for uncertainties in projecting non-sulphate aerosols and greenhouse gas
concentrations. [Based upon (a) Chapter
3, Figure 3.12,
(b) Chapter 3, Figure
3.12, (c) Chapter 5, Figure
5.13, (d) Chapter 9, Figure
9.14, (e) Chapter 11,
Figure 11.12, Appendix
II

Models have been used to make projections
of atmospheric concentrations of greenhouse gases and aerosols, and hence of future
climate, based upon emissions scenarios from the IPCC Special Report on Emission
Scenarios (SRES) (Figure 5). These scenarios
were developed to update the IS92 series, which were used in the SAR and are shown
for comparison here in some cases.

Greenhouse gases

Emissions of CO2 due to fossil fuel burning are virtually certain7
to be the dominant influence on the trends in atmospheric CO2 concentration
during the 21st century.

As the CO2 concentration of the atmosphere increases, ocean and
land will take up a decreasing fraction of anthropogenic CO2 emissions.
The net effect of land and ocean climate feedbacks as indicated by models
is to further increase projected atmospheric CO2 concentrations,
by reducing both the ocean and land uptake of CO2.

By 2100, carbon cycle models project atmospheric CO2 concentrations
of 540 to 970 ppm for the illustrative SRES scenarios (90 to 250% above the
concentration of 280 ppm in the year 1750), Figure
5b. These projections include the land and ocean climate feedbacks. Uncertainties,
especially about the magnitude of the climate feedback from the terrestrial
biosphere, cause a variation of about -10 to +30% around each scenario. The
total range is 490 to 1260 ppm (75 to 350% above the 1750 concentration).

Changing land use could influence atmospheric CO2 concentration.
Hypothetically, if all of the carbon released by historical land-use changes
could be restored to the terrestrial biosphere over the course of the century
(e.g., by reforestation), CO2 concentration would be reduced by
40 to 70 ppm.

Model calculations of the concentrations of the non-CO2 greenhouse
gases by 2100 vary considerably across the SRES illustrative scenarios, with
CH4 changing by –190 to +1,970 ppb (present concentration 1,760
ppb), N2O changing by +38 to +144 ppb (present concentration 316
ppb), total tropospheric O3 changing by -12 to +62%, and a wide
range of changes in concentrations of HFCs, PFCs and SF6, all relative
to the year 2000. In some scenarios, total tropospheric O3 would
become as important a radiative forcing agent as CH4 and, over
much of the Northern Hemisphere, would threaten the attainment of current
air quality targets.

Reductions in greenhouse gas emissions and the gases that control their
concentration would be necessary to stabilise radiative forcing. For example,
for the most important anthropogenic greenhouse gas, carbon cycle models indicate
that stabilisation of atmospheric CO2 concentrations at 450, 650
or 1,000 ppm would require global anthropogenic CO2 emissions to
drop below 1990 levels, within a few decades, about a century, or about two
centuries, respectively, and continue to decrease steadily thereafter. Eventually
CO2 emissions would need to decline to a very small fraction of
current emissions.

Aerosols

The SRES scenarios include the possibility of either increases or decreases
in anthropogenic aerosols (e.g., sulphate aerosols (Figure
5c), biomass aerosols, black and organic carbon aerosols) depending on
the extent of fossil fuel use and policies to abate polluting emissions. In
addition, natural aerosols (e.g., sea salt, dust and emissions leading to
the production of sulphate and carbon aerosols) are projected to increase
as a result of changes in climate.

Radiative forcing over the 21st century

For the SRES illustrative scenarios, relative to the year 2000, the global
mean radiative forcing due to greenhouse gases continues to increase through
the 21st century, with the fraction due to CO2 projected to increase
from slightly more than half to about three quarters. The change in the direct
plus indirect aerosol radiative forcing is projected to be smaller in magnitude
than that of CO2.

Global average temperature and sea level are projected to rise under all IPCC
SRES scenarios.

In order to make projections of future climate, models incorporate past, as well
as future emissions of greenhouse gases and aerosols. Hence, they include estimates
of warming to date and the commitment to future warming from past emissions.

Temperature

The globally averaged surface temperature is projected to increase by 1.4
to 5.8°C (Figure 5d) over the period
1990 to 2100. These results are for the full range of 35 SRES scenarios, based
on a number of climate models10,
11.

Temperature increases are projected to be greater than those in the SAR,
which were about 1.0 to 3.5°C based on the six IS92 scenarios. The higher
projected temperatures and the wider range are due primarily to the lower
projected sulphur dioxide emissions in the SRES scenarios relative to the
IS92 scenarios.

The projected rate of warming is much larger than the observed changes during
the 20th century and is very likely7
to be without precedent during at least the last 10,000 years, based on palaeoclimate
data.

By 2100, the range in the surface temperature response across the group
of climate models run with a given scenario is comparable to the range obtained
from a single model run with the different SRES scenarios.

On timescales of a few decades, the current observed rate of warming can
be used to constrain the projected response to a given emissions scenario
despite uncertainty in climate sensitivity. This approach suggests that anthropogenic
warming is likely7
to lie in the range of 0.1 to 0.2°C per decade over the next few decades
under the IS92a scenario, similar to the corresponding range of projections
of the simple model used in Figure 5d.

Based on recent global model simulations, it is very likely7
that nearly all land areas will warm more rapidly than the global average,
particularly those at northern high latitudes in the cold season. Most notable
of these is the warming in the northern regions of North America, and northern
and central Asia, which exceeds global mean warming in each model by more
than 40%. In contrast, the warming is less than the global mean change in
south and southeast Asia in summer and in southern South America in winter.

Recent trends for surface temperature to become more El Niño-like
in the tropical Pacific, with the eastern tropical Pacific warming more than
the western tropical Pacific, with a corresponding eastward shift of precipitation,
are projected to continue in many models.

Precipitation

Based on global model simulations and for a wide range of scenarios, global
average water vapour concentration and precipitation are projected to increase
during the 21st century. By the second half of the 21st century, it is likely7
that precipitation will have increased over northern mid- to high latitudes
and Antarctica in winter. At low latitudes there are both regional increases
and decreases over land areas. Larger year to year variations in precipitation
are very likely7
over most areas where an increase in mean precipitation is projected.

Extreme Events

Table 1 depicts an assessment of confidence
in observed changes in extremes of weather and climate during the latter half
of the 20th century (left column) and in projected changes during the 21st century
(right column)a. This assessment relies on observational and modelling
studies, as well as the physical
plausibility of future projections across all commonly-used scenarios and is
based on expert judgement
7.

For some other extreme phenomena, many of which may have important impacts
on the environment and society, there is currently insufficient information
to assess recent trends, and climate models currently lack the spatial detail
required to make confident projections. For example, very
small-scale phenomena, such as thunderstorms, tornadoes, hail and lightning,
are not simulated in climate models.

Table 1: Estimates of confidence in observed and
projected changes in extreme weather and climate events.

a For more details
see Chapter 2 (observations)
and Chapter 9, 10
(projections).b For other areas, there are either
insufficient data or conflicting analyses.c Past
and future changes in tropical cyclone location and frequency are uncertain.

El Niño

Confidence in projections of changes in future frequency, amplitude, and
spatial pattern of El Niño events in the tropical Pacific is tempered
by some shortcomings in how well El Niño is simulated in complex models.
Current projections show little change or a small increase in amplitude for
El Niño events over the next 100 years.

Even with little or no change in El Niño amplitude, global warming
is likely7
to lead to greater extremes of drying and heavy rainfall and increase the
risk of droughts and floods that occur with El Niño events in many
different regions.

Monsoons

It is likely
7 that warming associated with increasing greenhouse gas
concentrations will cause an increase of Asian summer monsoon precipitation
variability. Changes in monsoon mean duration and strength depend on the details
of the emission scenario. The confidence in such projections is also limited
by how well the climate models simulate the detailed seasonal evolution of
the monsoons.

Thermohaline circulation

Most models show weakening of the ocean thermohaline circulation which leads
to a reduction of the heat transport into high latitudes of the Northern Hemisphere.
However, even in models where the thermohaline circulation weakens, there
is still a warming over Europe due to increased greenhouse gases. The current
projections using climate models do not exhibit a complete shut-down of the
thermohaline circulation by 2100. Beyond 2100, the thermohaline circulation
could completely, and possibly irreversibly, shut-down in either hemisphere
if the change in radiative forcing is large enough and applied long enough.

Snow and ice

Glaciers and ice caps are projected to continue their widespread retreat
during the 21st century.

The Antarctic ice sheet is likely
7 to gain mass because of greater precipitation, while the
Greenland ice sheet is likely7 to lose mass because the increase in runoff
will exceed the precipitation increase.

Concerns have been expressed about the stability of the West Antarctic ice
sheet because it is grounded below sea level. However, loss of grounded ice
leading to substantial sea level rise from this source is now widely agreed
to be very unlikely
7 during the 21st century, although its dynamics are still
inadequately understood, especially for projections on longer time-scales.

Sea level

Global mean sea level is projected to rise by 0.09 to 0.88 metres between
1990 and 2100, for the full range of SRES scenarios. This is due primarily
to thermal expansion and loss of mass from glaciers and ice caps (Figure
5e). The range of sea level rise presented in the SAR was 0.13 to 0.94
metres based on the IS92 scenarios. Despite the higher temperature change
projections in this assessment, the sea level projections are slightly lower,
primarily due to the use of improved models, which give a smaller contribution
from glaciers and ice sheets.

Anthropogenic climate change will persist for many centuries.

Emissions of long-lived greenhouse gases (i.e., CO2, N2O,
PFCs, SF6) have a lasting effect on atmospheric composition, radiative
forcing and climate. For example, several centuries after CO2 emissions
occur, about a quarter of the increase in CO2 concentration caused
by these emissions is still present in the atmosphere.

After greenhouse gas concentrations have stabilised, global average surface
temperatures would rise at a rate of only a few tenths of a degree per century
rather than several degrees per century as projected for the 21st century
without stabilisation. The lower the level at which concentrations are stabilised,
the smaller the total temperature change.

Global mean surface temperature increases and rising sea level from thermal
expansion of the ocean are projected to continue for hundreds of years after
stabilisation of greenhouse gas concentrations (even at present levels), owing
to the long timescales on which the deep ocean adjusts to climate change.

Ice sheets will continue to react to climate warming and contribute to sea
level rise for thousands of years after climate has been stabilised. Climate
models indicate that the local warming over Greenland is likely7 to be one
to three times the global average. Ice sheet models project that a local warming
of larger than 3°C, if sustained for millennia, would lead to virtually
a complete melting of the Greenland ice sheet with a resulting sea level rise
of about 7 metres. A local warming of 5.5°C, if sustained for 1,000 years,
would be likely7 to result in a contribution from Greenland of about 3 metres
to sea level rise.

Current ice dynamic models suggest that the West Antarctic ice sheet could
contribute up to 3 metres to sea level rise over the next 1,000 years, but
such results are strongly dependent on model assumptions regarding climate
change scenarios, ice dynamics and other factors.

Further action is required to address remaining gaps in information and understanding.

Further research is required to improve the ability to detect, attribute and
understand climate change, to reduce uncertainties and to project future climate
changes. In particular, there is a need for additional systematic and sustained
observations, modelling and process studies. A serious concern is the decline
of observational networks. The following are high priority areas for action.

Systematic observations and reconstructions:

Reverse the decline of observational networks in many parts of the
world.

Sustain and expand the observational foundation for climate studies
by providing accurate, long-term, consistent data including implementation
of a strategy for integrated global observations.

Enhance the development of reconstructions of past climate periods.

Improve the observations of the spatial distribution of greenhouse gases
and aerosols.

Modelling and process studies:

Improve understanding of the mechanisms and factors leading to changes
in radiative forcing.

Understand and characterise the important unresolved processes and feedbacks,
both physical and biogeochemical, in the climate system.

Improve the integrated hierarchy of global and regional climate models
with a focus on the simulation of climate variability, regional climate
changes and extreme events.

Link more effectively models of the physical climate and the biogeochemical
system, and in turn improve coupling with descriptions of human activities.

Cutting across these foci are crucial needs associated with strengthening international
co-operation and co-ordination in order to better utilise scientific, computational
and observational resources. This should also promote the free exchange of data
among scientists. A special need is to increase the observational and research
capacities in many regions, particularly in developing countries. Finally, as
is the goal of this assessment, there is a continuing imperative to communicate
research advances in terms that are relevant to decision making.

The Emissions Scenarios of the Special Report on Emissions
Scenarios (SRES)

A1. The A1 storyline and scenario family describes a future world of
very rapid economic growth, global population that peaks in mid-century
and declines thereafter, and the rapid introduction of new and more efficient
technologies. Major underlying themes are convergence among regions, capacity
building and increased cultural and social interactions, with a substantial
reduction in regional differences in per capita income. The A1 scenario
family develops into three groups that describe alternative directions
of technological change in the energy system. The three A1 groups are
distinguished by their technological emphasis: fossil intensive (A1FI),
non-fossil energy sources (A1T), or a balance across all sources (A1B)
(where balanced is defined as not relying too heavily on one particular
energy source, on the assumption that similar improvement rates apply
to all energy supply and end use technologies).

A2. The A2 storyline and scenario family describes a very heterogeneous
world. The underlying theme is self-reliance and preservation of local
identities. Fertility patterns across regions converge very slowly, which
results in continuously increasing population. Economic development is
primarily regionally oriented and per capita economic growth and technological
change more fragmented and slower than other storylines.

B1. The B1 storyline and scenario family describes a convergent world
with the same global population, that peaks in midcentury and declines
thereafter, as in the A1 storyline, but with rapid change in economic
structures toward a service and information economy, with reductions in
material intensity and the introduction of clean and resource-efficient
technologies. The emphasis is on global solutions to economic, social
and environmental sustainability, including improved equity, but without
additional climate initiatives.

B2. The B2 storyline and scenario family describes a world in which the
emphasis is on local solutions to economic, social and environmental sustainability.
It is a world with continuously increasing global population, at a rate
lower than A2, intermediate levels of economic evelopment, and less rapid
and more diverse technological change than in the B1 and A1 storylines.
While the scenario is also oriented towards environmental protection and
social equity, it focuses on local and regional levels.

An illustrative scenario was chosen for each of the six scenario groups
A1B, A1FI, A1T, A2, B1 and B2. All should be considered equally sound.

The SRES scenarios do not include additional climate initiatives, which
means that no scenarios are included that explicitly assume implementation
of the United Nations Framework Convention on Climate Change or the emissions
targets of the Kyoto Protocol.